In exhaust gas discharged from a lean-burn engine such as a boiler, a gas turbine, a lean burn-type gasoline engine, or a diesel engine, various harmful substances derived from fuel or combustion air are included depending on a structure and a type thereof. Such harmful substances include a hydrocarbon (HC) a soluble organic fraction (it may also be called SOF), soot, carbon monoxide (CO), and nitrogen oxides (NOx), and they are regulated by the Air Pollution Control Law. In addition, as a purification method thereof, there has been practically used a contact treatment method for purifying exhaust gas by making it contacted with a catalyst.
In addition, in such a lean burn engine, there may be the case where generation amount of the harmful substances such as CO and THC, which are incomplete combustion substances, is controlled by controlling combustion temperature, by operation such as supplying air of optimum amount for combustion, depending on kinds or supply amount of fuel; while, there may also be the case of incurring generation of NOx by increased combustion temperature. Such a state is similar also in an internal combustion engine, and in the case of a diesel engine, because of having a structure for operating an engine by lean-burn, the nitrogen oxides are easily discharged. Among them, in the case where a diesel engine loaded on an automobile, because operation condition thereof is always changing, it was extremely difficult to suitably suppress generation of the harmful substances.
As a purification method of the harmful substances discharged in this way, such a method has been widely investigated that installs a catalyst at an exhaust gas passage. There has been proposed a system of an oxidation catalyst aiming at purification of mainly SOP components; a catalyst aiming at oxidizing and purifying unburned gas components; or a catalyst for trapping, oxidizing and purifying the soot component, in combining them with a filter; although the catalyst to be used differs depending on the harmful components discharged from an engine, or an exhaust gas regulation. In these catalysts, a noble metal such as mainly Pt or Pd has been used to promote an oxidation reaction. Because the diesel engine has relatively larger displacement and more exhaust gas amount flown out, as compared with a gasoline engine or the like, volume of the catalyst also necessarily becomes larger, and amount of the noble metal to be used becomes more, to obtain sufficient purification performance, as compared with a catalyst for a gasoline engine or the like. A catalyst for the gasoline engine, where regulations of exhaust gas from an automobile have preceded, has also used the noble metal components conventionally, and accompanying with strengthening of exhaust gas regulations for the diesel engine, such a situation has appeared that a relatively rare and expensive noble metal, among resources on the earth, has been used more and more.
Additionally, accompanying with the exhaust gas regulation of NOx, a catalyst system using a NOx storage catalyst or a selective catalytic reduction (hereafter it may also be referred to as SCR) catalyst has also been proposed, as a NOx purification catalyst. As for the SCR catalyst, several types of reducing agents to be used for purification of NOx have been known, and in the SCR, where the NH3 component is used as a reducing agent, NOx is finally reduced to N2 mainly by the following reaction formulas (1) to (3):4NO+4NH3+O2→4N2+6H2O  (1)6NO2+8NH3+O2→7N2+12H2O  (2)NO+NO2+2NH3→2N2+3H2O  (3)
In a denitration catalyst system utilizing such a reaction mechanism, a gasified NH3 may be used as the reducing component, however, NH3 itself has irritating odor or hazardous property. Therefore, there has been proposed a system for adding urea water, as the NH3 component, from the upstream of the denitration catalyst, generating NH3 by pyrolysis or hydrolysis, and having this acted as a reducing agent to exert denitration performance by the above reaction formulas.
Reaction formulas for obtaining NH3 by decomposition of urea in this way are as follows:NH3—CO—NH2→NH3+HCNO(pyrolysis of urea)HCNO+H2O→NH3+CO2(hydrolysis of isocyanic acid)NH2—CO—NH2+H2O→2NH3+CO2(hydrolysis of urea)
In purification of NOx in exhaust gas, it is ideal that NH3 supplied is all consumed in the above denitration reactions (1) to (3). However, in NOx purification in a vehicle loaded with a diesel engine, where transient operation condition is predicted under practical running condition, it is also predicted to use NH3, which was intentionally made adsorbed on the SCR catalyst surface by supplying an surplus reducing agent than that consumed in the reaction, so as to be able to effectively purify NOx under any running condition. In this way, when exhaust gas temperature is raised rapidly by rapid acceleration in a state that NH3 is adsorbed on the SCR catalyst or the like, NH3 eliminated does not contribute to the NOx purification reaction, and leaks to the downstream of the SCR catalyst (hereafter it may be referred to as slip, or NH3 slip), where a risk of incurring secondary pollution, such as new environmental contamination, has been pointed out.
As a countermeasure against such a problem, it is also considered to increase capacity of the SCR to a degree not requiring NOx purification using NH3 adsorbed, however, because of limitation in loading capacity or arrangement of the catalyst in an automotive application, such countermeasures cannot be said a practical solution that simply increases catalyst capacity of the SCR.
Other than this, various catalyst technologies have been investigated, as a treatment system after the addition of other catalysts, without using only the SCR catalyst (for example, refer to PATENT DOCUMENT 1 and PATENT DOCUMENT 4). In addition, there has also been investigated a purification method for slipped NH3 by oxidation as in the following reaction formula (4), by installing a NH3 purification catalyst, where platinum (Pt), palladium (Pd), rhodium (Ph) or the like is supported on a base material such as alumina, at the latter part of the SCR, to purify NH3 slipped from the SCR.2NH3+3/2O2→N2+3H2O  (4)
However, because the above, catalyst for purifying NH3 uses a noble metal component such as platinum, palladium, or rhodium, having high oxidation performance, as a catalyst active species, there was a problem of incurring new generation of NOx components such as N2O, NO, and NO2 at the same time of oxidation of NH3, as shown in the following reaction formulas (5) to (7).2NH3+5/2O2→2NO+3H2O  (5)2NH3+7/2O2→2NO2+3H2O  (6)2NH3+2O2→N2O+3H2O  (7)
To suppress generation of such NOx, there has been proposed a purification catalyst arranged with a component having NH3 oxidative decomposition activity at the lower layer, and arranged with a denitration component at the upper layer (refer to PATENT DOCUMENT 5) This is understood as a catalyst capable of not only purifying NH3 by oxidation of NH3 but also having a role of a NOx purification reaction by reacting NOx generated by NH3 oxidation of the above reaction formulas (5) to (7), with slipped NH3 not yet used in the oxidation reaction. There have also been proposed a catalyst for exhaust gas purification using one or more kind of oxides selected from titanium, tungsten, molybdenum or vanadium, as a denitration component of the upper layer refer to PATENT DOCUMENT 2); or a catalyst for ammonia oxidative decomposition catalyst using a mixed system of a Ce—Ti—SO4—Zr-type component and a Fe—Si—Al oxide-type component at the upper layer (refer to PATENT DOCUMENT 3); and a purification catalyst using Fe-containing zeolite or Ce-containing zeolite at the upper layer (refer to PATENT DOCUMENT 6). Also in these catalysts having a role of NH3 purification, a noble metal is used as a NH3 oxidative component.
As described above, in a situation of an ever strengthening exhaust gas regulation year by year, ratio of loading an exhaust gas purification catalyst system on a vehicle has increased, and price of a noble metal has soared upwards under a situation of using a rare and expensive noble metal in a large quantity. On the other hand, a too expensive catalyst as the automotive exhaust gas purification catalyst is not practical, due to giving one factor of raising vehicle price, and thus purification technology using an inexpensive active component has been investigated, so that sufficient purification performance can be exerted by less usage of the noble metal.
There has been proposed, for example, in a catalyst system composed of a noble metal particles, a catalyst promoter component, and a substrate, formation of composite micro particles in a state that a noble metal salt and a metal salt exist at the same time, inside the micelle of the catalyst, using a reversed micelle method, so that contact area between the noble metal particles and the promoter component does not decrease by sintering, by which there has been described that promoter effect, which a metal compound has, becomes exerted, and a low cost catalyst having high catalytic activity and high heat resistance can be obtained (refer to PATENT DOCUMENT 7). In addition, there have been performed many investigations, for example, on exhaust gas catalysts using Au, as a substitution metal of a platinum group element in an automotive catalyst. There has been reported supporting of Au on a substrate consisting of a ceria-zirconia solid solution having a ceria content of 40 to 80% by weight, for example, as a exhaust gas purification catalyst consisting of an Au catalyst having high CO oxidation activity (refer to PATENT DOCUMENT 8).
Under such circumstance, there has been required a slipped NH3 purification catalyst, having also suppressing function of generation of N2O or NOx, which is capable of decreasing usage of the noble metal of the catalyst aiming at purification of slipped NH3, in the SCR catalyst system using the above NH3 as a reducing agent.
In particular such a catalyst has been earnestly desired that exerts high purification performance of NH3 at lower temperature, because temperature of exhaust gas is lowered during passing through inside a piping, and temperature of a catalyst bed of a slipped NH3 purification catalyst is lowered, due to use of the slipped NH3 purification catalyst at the most end part of the exhaust gas purification apparatus.